To improve performance and efficiency, future internal combustion engines will operate at higher temperatures and pressures than present-day engines. For example, commercial diesel engines may operate at cylinder temperatures of about 760.degree. C. (1400.degree. F.) to about 870.degree. C. (1600.degree. F.) and brake mean effective pressures up to about 1030 kPa (150 psi). Military diesel engines may operate at cylinder temperatures up to about 925.degree. C. (1700.degree. F) and brake mean effective pressures greater than about 1380 kPa (200 psi). Such conditions, combined with rapid thermal cycling induced by the cylinder firing cycle, create a severe environment for in-cylinder engine parts. To operate under such conditions, critical engine parts must be insulated. Insulation lowers the temperature of the parts and reduces the amount of heat rejected to the environment. To be cost effective, the insulation should have a service life greater than about 20,000 hours.
U.S. Pat. No. 4,738,227 to Kamo et al. describes a two-layer thermal barrier coating for insulating parts in internal combustion engines. The coating includes a base layer of zirconia (ZrO.sub.2) plasma sprayed over a metal engine part. The ZrO.sub.2 layer is covered with a layer of a wear resistant ceramic to improve its serveice life. Suitable wear resistant ceramics include one containing silica (SiO.sub.2), chromia (Cr.sub.2 O.sub.3) and alumina (Al.sub.2 O.sub.3) and another based on zircon (ZrSiO.sub.4).
U.S. Pat. No. 4,711,208 to Sander et al. discloses coating piston heads with several layers of flame or plasma sprayed material. The layers can include ZrO.sub.3, ZrSiO.sub.4, metal, and cermet. Sander et al. also teach that an aluminum titanate piston crown insert covered with a fully stabilized ZrO.sub.2 coating can replace the multilayered insulation.
Similar, multilayered, ceramic thermal barrier coatings are used in the aerospace industry to insulate turbine blades in gas turbine engines. Gas turbine engine parts, however, are not subjected to rapid thermal cycling as are internal combustion engine parts. Commonly assigned U.S. Pat. Nos. 4,481,237 to Bosshart et al. and 4,588,607 to Matarese et al. teach coatings that include a metallic bond coat deposited on a metal substrate, a metal/ceramic layer deposited on the bond coat, and a ZrO.sub.2 ceramic top layer deposited on the metal/ceramic layer.
Although ZrO.sub.2 -based thermal barrier coatings allow internal combustion engines to operate under severe conditions, to date, they have not achieved the desired service life. Therefore, what is needed in the art is a thermal barrier coating that allows internal combustion engines to operate under severe conditions while achieving an acceptable service life.